OBJECTIVE: To compare Pit-1 transcription factor splice variants in primates and rodents. RESULTS The Pit-1 transcription factor is expressed in the vertebrate pituitary gland, and is critical for the determination of somatotropes, lactotropes and thyrotropes. We recently also demonstrated that Pit-1 and Pit-1 beta variants are expressed in the rhesus and human placenta. In the course of the these studies, we identified a novel splice variant in the rhesus pituitary gland which we named Pit-1 gamma. This variant had a novel splice donor and acceptor pattern not previously described. Because this region of Pit-1 is responsible for gene-selective transcriptional activation, we wished to determine if this splice pattern is unique to the primate. We obtained fetal and adult mouse tissues, and as with the rhesus pituitary gland, a novel pattern of splicing was seen with the rodent tissues. However, the intronic sequences which are utilized in generating the splice variants in the mouse are highly divergent from those in the primate and thus the rodent peptide insertion is highly divergent from that in the rhesus monkey and human. This divergent exon I/exon II junction sequence is likely to have significantly different functional transactivation properties compared to the well-defined Pit-1 and Pit-1 beta molecules. FUTURE DIRECTIONS To determine the splicing pattern in human pituitary tissue, and to define the ontogeny and relative levels of rodent Pit-1 gamma-like mRNA during development. KEY WORDS pituitary gland, mRNA splicing, homeobox, pit-1 Core)